1. Field of the Invention
This invention relates to high-temperature direct-contact thermal energy storage. The thermal energy stored according to this invention is in the temperature range of over about 400.degree. F. up to about 3000.degree. F. and is directly stored, without necessity of heat exchange tubes to provide containment and heat transfer surface, in composite latent/sensible heat thermal energy storage media utilizing the heat of fusion and high temperature stability of alkali metal and alkaline earth carbonates, chlorides, nitrates, nitrites, fluorides, hydroxides, sulfates and mixtures thereof and metals and alloys maintained within a porous storage-support material which itself is capable of storage of sensible heat. Various mixtures of alkali metal and alkaline earth salts and various metals and alloys may be used to obtain desired thermal and physical properties for storage of thermal energy derived from solar energy sources, industrial waste and process heat, high temperature gas reactors and the like.
2. Description of the Prior Art
There have been many prior attempts to store thermal energy utilizing the heat of fusion in aqueous-hydrate systems. For example, U.S. Pat. No. 3,986,969 teaches the use of a heat of fusion material plus attapulgite clay as a homogenizing agent; U.S. Pat. No. 1,894,775 teaches various latent heat of fusion storage chemicals; U.S. Pat. No. 4,146,057 teaches thermal storage as latent heat of fushion by passing a closed potassium loop and a closed steam/water loop through aluminum which serves to store the latent heat of fusion and is exemplary of prior practices utilizing tube heat exchangers to provide containment and heat transfer surface between storage media and working fluid; U.S. Pat. No. 4,223,721 teaches thermal storage by a eutectic salt packed with a thermal insulating material such as glass fiber insulation; and Japanese Pat. No. 51-96788 teaches thermal storage at about room temperature by hydrate reaction of Na.sub.2 SO.sub.4 and/or Na.sub.2 CO.sub.3 with a light aggregate such as gypsum for support and prevention of deliquescence or efflorescence. U.S. Pat. No. 4,237,023 teaches an aqueous heat storage composition which absorbs and stores heat as it is heated above its phase-change temperature and releases stored heat as it is cooled below its phase-change temperature, including use of fumed silicon dioxide which acts as a stabilizing agent and provides prolonged heat storage efficiency. U.S. Pat. No. 3,720,198 teaches thermal storage by heat of fusion with seed crystals in the thermal storage material to prevent change of distribution during the melting phase. The prior art hydrate systems have experienced problems with supercooling and phase separation which the last two patents referred to seek to overcome.
Latent heat based thermal energy storage systems have employed mechanical scrapers to remove solidified salts from finned tube heat exchangers during discharge cycles to improve thermal efficiencies of such systems. However, the mechanical systems have a limited lifetime, require frequent maintenance, and may only be used on the discharge cycle due to scraper freeze-up. Further, tube-intensive latent heat thermal energy systems require high cost super alloy materials for adequate strength and corrosion resistance above about 1300.degree. F.